Electric capacitor



Dec. 15, 1942. F, C RK 2,305,580

ELECTRIC CAPACITOR Filed Aug. 9, 1941 PHOSPHATE COATED IMPREGNATED WITHLIQUID DIELECTRIC MATERIAL IMPREGNATED WITH LIQUID DIELECTRIC MATERIALFig.3.

PHOSPHATE COATING Inventor:

is Attorney Frank M. Clark,

Patented Dec. 15, 1942 ELECTRIC CAPACITOR Frank M. Clark, Plttsfleld,Mass., assignor to General Electric Company, a corporation of New YorkApplication August 9, 1941, Serial No. 406,205

4 Claims.

This invention relates to electric capacitors, and its main object is toprovide dielectric elements therefor which consist at least in part ofnon-conducting phosphate of a metal as, for example, an iron or zincphosphate.

One of the objects of my invention is to provide electrical capacitorswhich will embody high elec- A capacitor fundamentally employs closelyjuxtaposed armatures of conducting material which ordinarily consist ofmetal foil or other form of thin metal. The armatures commonly areseparated by one or more thin sheets of dielectric material, commonlyconsisting of paper. In exceptional instances, as in electrolyticcapacitors,

the dielectric element is constituted by a film or coating of oxideformed on the surface of one or both of the armatures.

In accordance with the present invention, a coating of film of aphosphate of the foundation metal is chemically formed on a capacitorarmature. This film is characterized by having a high degree ofdielectric continuity, freedom from pcrosity and freedom from conductingareas, and unexpectedly favorable dielectric properties as willhereinafter appear.

In producing a phosphate-coated electrode in accordance with myinvention, I may use any electrode metal which is capable of forming acontinuous dielectric film on its surfac by chemical reaction withphosphoric acid or an inorganic phosphate. Zinc is of practical utilitysince it can be obtained commercially as foil of the desired thinness.Zinc foil as thin as .0003" to .0005" is suitable. Iron also issuitable. In some cases, iron, zinc, or other suitable metal may beelectro-deposited or otherwise coatedon a foundation of metal nototherwise so favorable. Coppef may be thus coated and converted intofoil.

A metal to be phosphate-coated is treated with of phosphoric acid and1.5 grams of manganese dioxide per liter, for example, by simpleimmersion of the metal in the solution or by passing an elongatedarticle, such as a strip of thin foil, through a reaction bath. Thelength of the immersion time is dependent upon the type of film desiredand may vary from five to sixty minutes. Another aqueous treatingsolution which has been foundto be of utility consists of ammoniumphosphate (NH4)HPO4. lution containing 10 per cent concentration of thesalt at a solution temperature of about 95 to 100 C. is employed. Zincfoil immersed as described in such a solution for five minutes has beenfound to be satisfactorily coated with a inorganic dielectric layer.

In some instances it has been found possible to accelerate the chemicalreactions involved by the adition of from one to two per cent of zincnitrate or a small trace (not over five-tenths per cent) of colloidalcopper.

A further illustration of a suitable reaction solution is one made froma phosphate mixture obtainable in the market as "Parko mixture." Thissolution consistsof phosphoric acid and an acid phosphate to which asmall amount of zinc nitrate is added. The reaction solution ismaintained at a pH value of 2 to 2.5. When zinc or iron foil is immersedin this solution at a temperature from 90 C. to 100 0., chemicalreaction occurs. The metal surface is coated with the desired dielectricfilm after approximately five minutes of immersion.

After a capacitor armature has been provided with a phosphate dielectricfilm using one of the reaction solutions described, it is washed withwater and dried. When treating armature foil for the usual rolled typeof capacitor, it has been found most convenient to perform the variousoperations by one continuous passage of the foil in an endless stripthrough first a reaction bath,

then through a scrubber containing water where the reacting solution isremoved, and finally through an oven where the foil is dried by contactwith a stream of hot, dry air.

The insulated strip of foil or the like is wound into rolls for laterfabrication in a capacitor assembly, as will be described hereinafter.

Referring to the drawing, as shown in Figs 1 and 3, the capacitorassembly may consist of juxtaposed insulated foils l, 2 assembled inclose contact without any separate spacing element, the reaction film 3(Fig. 3) on the surface of the electrodes constituting a dielectricmedium. A1-

a. boiling aqueous solution containing 25 grams 5 though a single coatedfoil is theoretically sum- Preferably, an aqueous 50- cient for a lowvoltage field to furnish the required dielectric element in combinationwith an uncoated foil, two juxtaposed coated foils are desirable from apractical standpoint because of the great difliculty in eliminating alltraces of weak spots or areas of dielectric imperfections from theinsulation formed on the foil surface.

Capacitors in which the dielectric consists solely of insulatingphosphate film can be operated with impressed potentials in a range ofvoltages up to 110v0lts. Such capacitors after assembly are dried undervacuum at 100 to 125. C. or higher, and are impregnated with adielectric material to eliminate air films, the presence of whichreduces electrical capacity. While various liquids and liquefied waxesor resins may be used, I prefer liquid pentachlor diphenyl as animpregnant. Capacitors impregnated with pentachlor diphenyl arecharacterized by a breakdown limit of approximately 350 volts and apower factor of less than .5 per cent at room temperature.

The electric terminals 4, 4' may be made, as shown in Fig. 3, by foldingan end portion of the armatures or by other known constructions. Fig. 3the phosphate coating 3 (indicating by stippling) is shown as partlyremoved (for the sake of contrast) from the foundation metal. Electriccontact may be made to the terminals 4, 4 even though they are coatedwith a thin layer of phosphate as it is easy to break through thephosphate with a pressure contact device. Other known contactconstructions may be used if care is exercised to remove or breakthrough the insulating phosphate film where required to make goodelectrical connection.

In some cases, as shown in Fig. 2, I prefer to employ capacitors havingphosphate-coated armatures prepared in accordance with this invention,separated from each other not only by the phosphate films, but also byone or more layers 1, 8 of some other dielectric material. For example,standard capacitor kraft paper tissue, preferably .0003" to .0005" inthickness, may be used. Other separators may be employed such,

for example, as a resinous material or a plastic such as celluloseacetate. Suitable resins are alkyd resins, polymerized styrene, orphenol formaldehyde resin. In Fig. 2, paired sheets 5, 6 and 1, 8 ofsuch dielectric material are illustrated although a different number ora single sheet may be used.

After assembly, such capacitors, with one or more spacing sheets, aredried at 100 C. to 120 C. under a vacuum, preferably lower than 500microns of mercury pressure and thereupon are impregnated with adielectrical liquid or solid as above indicated. Finally, they aremounted in a sealed container, as well understood.

Capacitors of untreated (normal surface) foil, when spaced with but onesheet of capacitor tissue; are of little practical value because ofunavoidable inherent defects in the paper spacer. Such defects includeiweak spots or shorts due to the presence of conducting particles(probably minute carbon or metal particles) which cause breakdown of thecapacitor under the application of but a few volts. In capacitorscontaining armatures coated with an insulating phosphate, the effect ofsuch weak spots in the paper is eliminated or reduced because of thepresence of the insulating film on the electrode surface which preventselectrical contact of the metallic conductor and the conducting materialof the weak spo in the paper.

A capacitor containing only one sheet of the capacitor tissue thusbecomes for the first time of practical value. A capacitor provided withthe combination of a phosphate film and a single sheet of paper afterdrying and impregnation with pentachlor diphenyl, has a breakdown valuein the range of 500 volts and is fully capable of use on commercialalternating current circuits at 220 or 330 volts. Such a capacitor ischaracterized by an electrical capacity of one microfarad for each fiveto six square feet of active electrode area. This is a reduction ofabout 20 to 30 per cent from th electrode area normally required for onemicrofarad of electrical capacity in capacitors of similar voltagerating.

It is in the dielectric .loss characteristic that another unexpected andhighly favorable property of capacitors incorporating my new type ofinsulated electrodes is observed. Normally, capacitors with electrodesof aluminum, tin, or zinc, spaced by a dielectric composed of multiplekraft capacitor tissue (.0003" to .001" per sheet) after vacuum dryingand impregnation with pentachlor diphenyl are characterized with a powerfactor of .3 to .4 per cent measured at 25 C. under a 60 cycle voltageat a stress of from 300 to 400 volts per mil.

With the increased electrical capacity per unit of electrode area, suchas is possessed by capacitors using myvimproved coated armatures, it isdesirable even more than ordinarily that the electrical losses should bereduced. The smaller surface area of the capacitor housing from whichthe heat generated in operation must be dissipated, coupled with an evenmoderately high power factor, would result in overheating of thecapacitor in service. Unexpectedly, my improved capacitor possesses thedesired low power factor, that is, low energy loss..

Capacitors utilizing foil or plate electrodes insulated with my newinsulating film, as already described, and separated by one sheet ofcapacitor tissue, are characterized by a power factor of .22 per centand in some instances as low as .20 per cent when tested at 25 C. undera 60 cycle voltage stress in the range of 300 to 400 volts. Thisdecrease in electrical losses is believed to be attributable to a moreintimate contact which is obtained between the impregnated dielectricand the electrode plate or foil as a result of the presence of theinsulating phosphate layer chemicall formed thereon as described.

When two sheets of capacitor tissue supplement the phosphate film, thebenefits of my invention are obtained in some measure and in additionthe breakdown value of the capacitor is correspondingly increased.Breakdown voltages in the range of 1800 volts to 2600 volts may berealized.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric capacitor comprising cooperating, closely juxtaposedarmatures, a film of phosphate of a metal chosen from the groupconsisting of iron and zinc functioning as a dielectric element betweensaid armatures and a second dielectric material filling voids betweensaid armatures.

2. An electric capacitor comprising cooperating, closely juxtaposedarmatures, a film of zinc phosphate interposed between said armaturesand functioning as a dielectric element, and a liquid dielectricmaterial filling voids between said armatures.

3. An electric capacitor comprising cooperating asoaoso 3 armatures ofzinc foil having thereon an adherent superficial layer of zincphosphate, paper interposed between said armatures, and a dielectricmaterial filling voids between said armatures.

4. An electric capacitor comprising juxtaposed armatures, at least oneof whichis provided with or zinc, said phosphate having dielectricproperties, paper interposed between said armatures, and a liquiddielectric material impregnating said paper and filling voids betweensaid armatures, said capacitor being adapted for operation under voltagestress of 300 to 400 volts per mil of dielectric with a power factor ofa fraction of one per cent.

FRANK M. CLARK.

